From a pictograph of a pestle; it was borrowed to express the central zodiac sign/horary sign, i.e. the middle part of the day, partly because the pestle was associated with striking the center of a mortar.

Noon (?), n. [AS. nn, orig., the ninth hour, fr. L.nona (sc. hora) the ninth hour, then applied to the church services (called nones) at that hour, the time of which was afterwards changed to noon. See Nine, and cf. Nones, Nunchion.]

1.

The middle of the day; midday; the time when the sun is in the meridian; twelve o'clock in the daytime.

2.

Hence, the highest point; culmination.

In the very noon of that brilliant life which was destined to be so soon, and so fatally, overshadowed.
Motley.